This invention relates in general to electrophotography and, in particular, to electrophotoconductive imaging members having multiple layers.
In electrophotography, an electrophotographic plate containing a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging its surface. The plate is then exposed to a pattern of activating electromagnetic radiation such as light. The radiation selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the electrophotograhic plate to a support such as a paper. This imaging process may be repeated many times with reusable photoconductive insulating layers.
An electrophotographic imaging member may be provided in a number of forms. For example, the imaging member may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. One type of composite imaging member comprises a layer of finely divided particles of a photoconductive inorganic compound dispersed in an electrically insulating organic resin binder. U.S. Pat. No. 4,265,990 discloses a layered photoreceptor having separate photogenerating and charge transport layers. The photogenerating layer is capable of photogenerating holes and injecting the photogenerated holes into the charge transport layer.
U.S Pat. No. 4,747,992 to Sypula et al discloses a process for fabricating a belt. The belt may be used for electrostatographic imaging members as a substrate layer. The substrate layer may comprise any of a number of film forming polymers, including polycarbonates, polysulfones, polyesters, and polyvinylfluoride. Electrostatographic imaging members are also disclosed which include photogenerating layers containing photoconductive compositions and/or pigments and a resinous binder. The basis for selection from among the materials for the charge generating layer and the substrate is not disclosed.
U.S. Pat. No. 4,756,993 to Kitatani et al discloses an electrophotographic photoreceptor comprising a light-transmitting conductive support comprised of a transparent thermoplastic resin film. The resins to be used for the conductive support include polyesters, polycarbonates, polyamides, acrylic resins, polyamide-imide resins, polystyrene, polyacetals, polyolefins, etc. An electrophotographic photosensitive layer is disclosed as being an organic photoconductive layer composed of an organic photoconductive substance. Polyvinylcarbazole and its derivatives are disclosed as one such organic photoconductive substance.
U.S. Pat. No. 4,895,784 to Shirai discloses a photoconductive member comprising a drum-shaped substrate and a photoconductive layer. The substrate may be either electroconductive or dielectric. Dielectric supports include films or sheets of synthetic resins, including polyester, polyethylene, polycarbonates, cellulose acetates, polypropylene, polyvinyl chloride, polyvinylidene chloride, polystyrene, polyamide, etc. The photoconductive layer may be a vacuum deposited layer comprising an amorphous material comprising silicon atoms.
U.S. Pat. No. 4,582,772 to Teuscher et al discloses layered photoconductive imaging devices comprising a substrate of organic polymeric material such as polycarbonates, polyamides and polyurethane. A photogenerating layer is provided comprising photoconductive particles or pigments randomly dispersed in a resinous binder. Sublimation of the photogenerating layer is not disclosed.
U.S. Pat. No. 4,702,980 to Matsuura et al discloses an electrostatic recording medium. The recording medium is a sheet-like product prepared from polyolefins.
As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, degradation of image quality was encountered during extended cycling. Moreover, complex, highly sophisticated duplicating and printing systems operating at very high speeds have placed stringent requirements including narrow operating limits on photoreceptors.
Modern composite imaging members have been developed having numerous layers which are highly flexible and exhibit predictable electrical characteristics within narrow operating limits to provide excellent images over many thousands of cycles. One type of multilayered photoreceptor that has been employed as a belt in electrophotographic imaging systems comprises a substrate, a conductive layer, a blocking layer, an adhesive layer, a charge generating layer, and a charge transport layer. This photoreceptor may also comprise additional layers such as an anti-curl layer and an optional overcoating layer.
During machine function, a photoconductive imaging member is constantly under repetitive electrophotographic cycling which subjects the electrically operative layers to high electrical charging/discharging cycles, multiple exposures to light for latent imaging development and erasure, and heat due to temperature elevation as a result of machine operation. The repetitive electrical and light fatigue lead to a gradual deterioration in the electrical characteristics of the imaging member, and limit its service life in the field. In the attempt to fabricate a robust photoconductive imaging system, many innovative ideas have been attempted with the intent to overcome this shortfall and extend the electrical functional life of the imaging member.
One of the most encouraging advances in photoconductive imaging development which has emerged in recent years is the successful fabrication of a novel design of a flexible imaging member which exhibits nearly ideal capacitive charging characteristics, outstanding photosensitivity, low electrical potential dark decay, and long term electrical cyclic stability. This novel imaging member design employed in belt form comprises a substrate, a conductive layer, a solution coated hole blocking layer, a solution coated adhesive layer, a thin vacuum sublimation deposited charge generating layer of pure organic pigment, a solution extruded charge transport layer with an adjacent solution co-extruded ground strip at one edge of the imaging layers, a solution extruded anti-curl layer, and an optional overcoating layer. For example,
U.S. Pat. No. 4,587,189 to Hor et al discloses photoconductive imaging members comprising a vacuum sublimation deposited benzimidazole perylene charge generating layer for photoelectric imaging and performance enhancement. This novel multilayered belt-imaging member provides excellent electrical properties and extended life, but is also seen to exhibit a major problem of charge generating layer mud-cracking. The observed charge generating layer mud-cracking consists of a two-dimensional network of cracks. Mud-cracking is believed by the present inventor to be the result of built-in internal stress in the charge generating layer due to the elevated temperature of the vacuum sublimation/deposition process and solvent penetration through the thin charge generating layer which dissolves the adhesive layer underneath during application of the charge transport layer solution.
Cracking in the charge generating layer has a serious impact on the versatility of a photoreceptor and reduces its practical value. Charge generating layer cracking not only can print out as defects, but may also act as stress concentration centers which then propagate the cracks into the other electrically operative layer, i.e., the charge transport layer, during dynamic belt machine cycling. In addition to the described mud-cracking problem, this novel imaging design also exhibits an inherent mechanical shortfall of low adhesion strength at the charge generating layer/adhesive layer interface. It has been observed that inadequate interfacial bond strength leads to frequent imaging layer delamination during dynamic fatigue belt flexing over small diameter supporting belt module rollers; that is, 19 mm diameter rollers.
While the above-mentioned imaging member gives the desirable electrical characteristic, there is an urgent need to reduce cracking in order to make the imaging member design acceptable for product implementation. It is also important to emphasize that any solution employed to solve the charge generating layer mud-cracking problem should produce no deleterious effects on the electrical and mechanical integrities of the original device.